1. Field of the Invention
The invention is related to arrangements for providing highly reliable timing signal distribution to a plurality of equipment units operating under a common system timing constraint. More particularly, the invention relates to fault-tolerant distribution of duplicated clock signals to a plurality of communication system equipment units.
2. Description of the Prior Art
Many prior art systems provide redundancy among various equipment entities to achieve high reliability. In equipment combinations such as electronic telephone switching systems, for example, critical timing sources are duplicated and arranged for fault-tolerant operation.
Examples of such fault-tolerant clock sources are described in U.S. Pat. Nos. 3,900,741--Fletcher et al., 3,965,432--Denenberg et al., and 4,025,874--Abbey. Such teachings deal with the provision of maintaining a clock source output in the presence of a disabling fault in one of the redundant sources. One approach, taught in U.S. Pat. Nos. 3,965,432 and in 4,025,874, utilizes a master/slave pair of clock sources. Another approach, as represented by U.S. Pat. No. 3,900,741, utilizes a predetermined quorum of a plurality of clock sources to furnish a fault-tolerant clock output.
In systems such as electronic time division multiplex PCM switching systems, the reliable distribution of the various timing signals also becomes an important requirement. The above cited relevant art does not address fault-tolerant signal distribution, but rather deals only with providing a fault-tolerant source of timing signals. There is therefore a need in the art for provision of fault-tolerant distribution, or bussing, of system clock signals among a plurality of system equipment units.